Conventionally, as a semiconductor device having a semiconductor stacked structure including an electron supply layer and an electron transit layer, a field-effect transistor for which GaN is used such as a high electron mobility transistor (HEMT) for which GaN is used (GaN-HEMT) is available.
As such a GaN-HEMT as just described, an HEMT is available in which AlGaN, InAlN, InAlGaN or the like is used for the electron supply layer and an oxide layer is formed by oxidizing part of the electron supply layer and besides a gate electrode is provided over the oxide layer.